The invention relates to a linear motion guide for mounting and longitudinally guiding machine parts, in which a first guide element provided to be fixed to a machine part is guided by a plurality of rolling elements along a longitudinal movement axis with respect to a second guide element, likewise provided to be fixed to a machine part, for this purpose each guide element has a groove-like recess running parallel to the longitudinal movement axis, boundary surfaces of the recess in each guide element are provided as rolling surfaces for the rolling elements, the rolling elements are arranged one after another in one or more rows in at least one cage between the two guide elements, the cage is provided with at least one first positive guide means, the cage with its first positive guide means being guided by second positive guide means fixed to the guide elements, each second positive guide means being arranged only on one of the guide elements and being fixed in position relative to the latter.
Linear motion guides of the generic type thus have at least one row of rolling elements, which roll in the grooves, always in a rectilinear movement. As distinct from recirculating linear motion guides, in the case of generic linear motion guides the rolling elements do not leave their rectilinear movement path, in which they are in contact with both guide elements in a load-bearing function. Linear motion guides of the above-described type are usually provided in order to permit relative movements of machine components.
In order that the rolling elements are always arranged at predetermined distances from one another, it has been known for a long time to arrange the rolling elements in a cage, as it is known. In a cage, a dedicated mount is provided for each rolling element, in which the rolling element can move in rotation. The linearly moving guide element produces a rotational movement in the rolling elements, from which a linear movement of the rolling elements also results, by means of which in turn the cage is carried along in a linear movement.
Intrinsically, the position of the cage and of the rolling elements with respect to the two guide elements is always predetermined. However, because of external influences, displacement of rolling elements with respect to their predetermined intended position may result. This can lead to a displacement of the cage, which in turn results in a change in the displacement range in which the two guide elements, or the machine components connected to the latter, can be displaced with respect to one another. In order to avoid such a movement, normally referred to as xe2x80x9ccage wanderingxe2x80x9d, there already exist various forms of positive guides for the cage.
For example, in a first previously disclosed positive guide, provision is made to connect a cage to a pinion. The pinion is located outside the two grooves, beside the two guide elements. The pinion engages in two racks, which are each fixed to one of the two machine parts which are intended to execute the relative movements. The unsatisfactory aspect of this solution is that, because of the required fixing of the racks to the machine parts, the manufacturer of the linear motion guide cannot supply the latter in preadjusted form. The racks have to be fitted by the machine manufacturer himself, and the individual guide components have to be toleranced relatively closely in order that the toothing system can function.
The same problem arises in the solution shown in U.S. Pat. No. 4,524,671. In this document, a linear guide based on rolling-contact bearings for a gun barrel with respect to a guide sleeve is shown. To this end, so-called strips are formed on the gun barrel and bearing rails are formed on the guide sleeve. The rolling elements are arranged in a cage in a spacing area between a strip and a bearing rail in each case. Furthermore, in order to guide the gun positively, in each case a rack, in which a pinion arranged on the cage engages, is fixed to the strip and the bearing rails. Both the pinion and the racks are located outside the spacing area provided for the rolling elements for one of the strips with respect to one of the bearing rails.
In another solution, previously disclosed, inter alia, by U.S. Pat. No. 5,427,454, in each case a rack is introduced into both guide elements, in the bottom of their grooves. A pinion connected to the cage is positively guided by both racks. In this case, it can be felt to be disadvantageous that the guide elements necessitate high fabrication expenditure, because of the racks to be introduced. In addition, the assembly of the guide proves to be difficult, since the position of the cage can no longer be changed as soon as the pinion engages in the racks. A further difficulty consists in the mutual alignment of the two racks arranged in the grooves, so that both racks engage correctly in the pinion. In addition, there is the risk of damaging teeth during assembly.
Finally, a further guide has been disclosed in which a cage belonging to the rolling elements is positively guided. Here, in each case a deflection roller is fitted at the two ends of the cage, in which roller a thin rope or a string is guided in each case. The ends of the two ropes are in each case fixed to the ends of the guide elements. This solution also entails complicated assembly. In addition, the required thin strings and the small deflection rollers are susceptible to defects. It has also been shown that, on account of external influences, such as temperature changes, length changes of the strings can occur. The length changes lead to play of the cage or even to distortions of the cage between the two strings. If the cage is guided by resilient spring elements instead of by strings, as shown in U.S. Pat. No. 4,262,974, then exact positive guidance can barely be achieved.
The invention is therefore based on the object of providing a linear motion guide which, with the most accurate guidance possible of the cage, exhibits favourable fabrication and assembly properties.
In the case of a device of the type mentioned at the beginning, this object is achieved, according to the invention, by the second positive guide means being fixed to the respective guide element outside the groove-like recesses, projecting between the two guide elements in order to guide the first positive guide means and here being operatively connected to the first positive guide means.
In a linear motion guide according to the invention, all the positive guide means can already be brought by the manufacturer into serviceable operative connection with one another, since all the positive guide means are fitted to the linear motion guide itself. This also results in no further components of the linear motion guide having to be fitted to the machine parts, apart from the guide elements themselves. Since the second positive guide means are fixed to external surfaces, such as side surfaces, of the guide elements, which side surfaces are easily accessible, preferably even in the assembled state of the linear motion guide, linear motion guides according to the invention are also comparatively simple to assemble. Of course, it would also be possible to fit each of the second positive guide means to an outer surface, located outside the groove, of one of the guide elements, the said surface lying opposite an outer surface of the other guide element in the assembled state of the linear motion guide. Expressed in other words, the second positive guide means could also be fixed to surfaces of the guide elements which bound a gap between the two guide elements.
An additional advantage is that, in order to form a linear motion guide according to the invention, use can be made of conventional guide elements which are relatively simple to fabricate. These guide elements merely have to be modified to the effect that fixing the second positive guide means to them is possible.
In the case of linear guides according to the invention, the second positive guide means should be fixed to their guide element in a fixed location, that is to say substantially immovably in relation to the second positive guide means. Immovable components can be assembled with comparatively little effort and are in addition little susceptible to defects in operation.
In the case of linear guides according to the invention, the positive guide means of the guide elements can preferably be intrinsically rigid, in order also to be able to absorb or to transmit loadings other than only pure tensile loadings, as is the case in the ropes previously known. The property xe2x80x9cintrinsically rigidxe2x80x9d is therefore to be understood as the opposite of the flexible ropes. In order to avoid damage to the positive guide means, it may be advantageous if these have a certain elasticity, in spite of their rigidity, in order to be able to absorb loadings by means of reversible deformation. As a result of the elasticity, premature fracture of the component can be avoided. The positive guide means can therefore advantageously be configured from an appropriate metallic material or from plastic.
In preferred embodiments according to the invention, the second positive guide means of the linear motion guides can each have an operative connecting part, which is operatively connected to the first positive guide means, the operative connecting part arranged in a fixed location on the guide element having a longitudinal extent which runs parallel to the longitudinal movement axis.
In this case, a maximum length of the two operative connecting parts in the displacement direction can correspond approximately to half, or slightly more than half, of the maximum displacement travel along which the two guide elements can be displaced in relation to each other. In the case of linear guides of the generic type based on rolling-contact elements, the cage always covers half the displacement travel executed by the two guide elements as a relative movement in relation to each other. If the length of the operative connecting parts is chosen to be slightly greater than half the displacement travel, a secure operative connection between the positive guide means can be ensured even at the end of the displacement travel.
In a preferred embodiment, the first positive guide means is a pinion fixed to the cage such that it can rotate about an axis of rotation. The pinion is preferably located in the centre of the cage. In particular when high forces act on the cage, a plurality of pinions can also be provided. If the positive guidance is based on the engagement of toothing systems, then the second guide means may be two racks, of which in each case one is fitted to one of the guide elements. This embodiment offers the advantage of high functional reliability of positive guides based on pinion/racks, without entailing the disadvantage of difficult assembly or of damaging teeth during the assembly.
In this case, it is further preferred for the racks to be designed as an angled profile in relation to a cross section, it being possible for the said profile to have a fixing leg and a toothed leg angled with respect thereto. The fixing leg is advantageously fitted to a freely accessible side surface of the corresponding guide element, running parallel to the longitudinal movement axis. The toothed leg provided as operative connecting part of the second positive guide means can engage in a gap between the two guide elements and can be operatively connected there to the second positive guide means.
If the racks on outer surfaces of the guide elements are in each case fitted to different sides of the two guide elements, then the two guide elements, including the fixing means required on the guide elements, can be configured to be completely structurally identical. This reduces the outlay on fabrication.
Although a positive guide based on tooth engagements is preferred according to the invention, instead of this, however, other form-fitting or else force-fitting positive guides can also be provided, such as positive guides using magnetic or inductive forces.
Further preferred refinements of the invention emerge from the claims and the drawing.